This is one of the classic problems of iPhone 6.
All of sudden, you would lose cellular service and it may or may not come back, sometimes reboot works, sometimes it doesn’t.
The reason is due to weak PCB design and flexible casing.
The problem is your baseband chip.
To confirm this problem, lets dial … 111.
Nah, dial *#06#
If nothing happens after you dial, it confirms the problem. If you see a long number displayed instead, good news, you are fine.
In this case, nothing happened. lets get to the baseband chip.
Baseband chip off…
reball it, put it back on…
Then dial *#06# again…
YAY! Problem fixed. IMEI displayed and now cellular service is back!